Add httpfs_client_implementation option, currently only default and httplib are valid

[carlopi]

Idea of this is allowing, in future versions of httpfs to offer a different version, namely curl, and possibly move the default.

Infrastructure would allow to register already in duckdb the option.

[carlopi]

My idea is that this could go into 1.3.2, and then post 1.3.2 we can consider merging #58 (more ready form in #77), adding curl as alternative implementation, and have users try it out / give feedback already via:

SET httpfs_client_implementation = curl;

Then in a follow up / after relevant testing we could consider changing the default.

[Tmonster]

Thanks. LGTM

[carlopi]

Question is like: merge + add to 1.3.2 or to be done later?

[Tmonster]

I don't really see it adding any functionality that is necessary for v1.3.2, but it also seems like a relatively safe PR, so not against it. If I had to choose, I would say wait until after v1.3.2 then merge it so it is in CORE_NIGHTLY

[carlopi]

Yes, it's minor, only minor advantage is the fact that setting will be there in any version of httpfs in 1.3.2, vs being an error in some cases and present in others, but I can see also the call for that, and that's your call.